The ability to model and predict multiaxial fatigue life without being too aggressive or conservative is of great interest to engineers. The purpose of this study was to examine the influence of the biaxiality ratio (σ2/σ1) on the multiaxial fatigue life of ductile metals. Fatigue models may exhibit significant variations in predicting the effect of the biaxiality ratio. Two notch specimens with varying biaxiality ratios were machined and experimentally tested to check the validity of two different multiaxial fatigue models, an equivalent stress-based model (PSP) and a critical plane model (DP). The results indicated that a life benefit exists with an increasing biaxiality ratio, but that equivalent stress model can overpredict fatigue life with higher biaxiality ratios with no third principal stress present. A new critical plane model was proposed to account for secondary stresses in fatigue life prediction, and showed good experimental correlation with its theoretical fatigue life predictions.